Configuration of coordinated multipoint transmission hypotheses for channel state information reporting

ABSTRACT

A system and method for providing an eNodeB with the flexibility to configure a Channel State Information (CSI) report to match a specific Coordinated Multipoint (CoMP) transmission hypothesis, which is a candidate for a downlink transmission to a User Equipment (UE) is disclosed. A UE receives, from the eNodeB, a configuration message that specifies a CSI report. The CSI report is specified by a particular interference hypothesis and a particular desired signal hypothesis corresponding to data transmission over at least one effective channel characterized by a specific reference signal. The UE estimates interference according to the interference hypothesis, and/or estimates at least one effective channel by performing measurements on the specific reference signal, and determines a CSI report based on the interference estimation and on the estimated effective channel. The UE also transmits the CSI report to the eNodeB.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/940,851, filed May 29, 2018, granted as U.S. Pat. No. 10,313,912 onJun. 4, 2019, which is a continuation of U.S. patent application Ser.No. 15/094,546, filed Apr. 8, 2016, granted as U.S. Pat. No. 9,961,582on May 1, 2018, which is a continuation of U.S. patent application Ser.No. 13/877,799, filed Apr. 4, 2013, granted as U.S. Pat. No. 9,337,970on May 10, 2016, which is a National Phase Entry of PCT/SE2013/050235,filed Mar. 13, 2013, which claims the benefit of U.S. ProvisionalApplication Ser. No. 61/612,920, filed Mar. 19, 2012, and entitled,“Configuration of Coordinated MultiPoint (CoMP) Transmission Hypothesesfor Channel State Information (CSI) Reporting,” all of which areexpressly incorporated herein by references in their entirety.

TECHNICAL FIELD

The present disclosure relates generally to wireless communicationssystems, and in particular to systems and methods for improving the linkadaptation in a wireless communications system.

BACKGROUND

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance isparticularly improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and/orrelated techniques are commonly referred to as MIMO.

The Long Term Evolution (LTE) standard, which is a standard defined bythe Third Generation Partnership Project (3GPP), is currently evolvingwith enhanced MIMO support. A core component in LTE is the support ofMIMO antenna deployments and MIMO related techniques. A current workingassumption in LTE-Advanced is the support of an 8-layer spatialmultiplexing mode, possibly with channel dependent precoding. The focusof the spatial multiplexing mode is to achieve high data rates infavorable channel conditions. An illustration of the spatialmultiplexing mode is provided in FIG. 1.

As seen in FIG. 1, the information carrying symbol vector s ismultiplied by an NT×r precoder matrix W_(N) _(T) _(×r), which serves todistribute the transmit energy in a subspace of the NT (corresponding toNT antenna ports) dimensional vector space. The precoder matrix istypically selected from a codebook of possible precoder matrices, andtypically indicated by means of a precoder matrix indicator (PMI). ThePMI specifies a unique precoder matrix in the codebook. If the precodermatrix is confined to have orthonormal columns, then the design of thecodebook of precoder matrices corresponds to a Grassmannian subspacepacking problem. Each of the r symbols in s corresponds to a layer and ris referred to as the transmission rank. In this way, spatialmultiplexing is achieved since multiple symbols can be transmittedsimultaneously over the same resource element (RE). The number ofsymbols r is typically adapted to suit the current channel properties.

LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in thedownlink, and Discrete Fourier Transform (DFT) precoded OFDM in theuplink. Therefore, the received NR×1 vector y_(n) for a certain resourceelement on subcarrier n (or alternatively data RE number n), assuming nointer-cell interference, is thus modeled by Equation (1)

y _(n) =H _(n) W _(N) _(T) _(×r) s _(n) +e _(n)  (1)

where e_(n) is a noise and interference vector obtained as realizationsof a random process. The precoder, W_(N) _(T) _(×r), can be a widebandprecoder, which is constant over frequency, or frequency selective.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel H, resulting in so-called channel dependentprecoding. This is also commonly referred to as closed-loop precodingand essentially strives for focusing the transmit energy into a subspacewhich is strong in the sense of conveying much of the transmitted energyto the UE. In addition, the precoder matrix may also be selected tostrive for orthogonalizing the channel. This means that the inter-layerinterference is reduced after proper linear equalization at the UE.

Channel State Information Reference Symbols (CSI-RS)

In LTE Release-10, a new reference symbol sequence (i.e., the CSI-RS)was introduced for estimating channel state information. The CSI-RSprovides several advantages over basing the CSI feedback on the commonreference symbols (CRS), as was done in previous releases of LTE. First,the CSI-RS is not used for demodulation of the data signal, and thusdoes not require the same density (i.e., the overhead of the CSI-RS issubstantially less). Second, CSI-RS provides a much more flexible meansto configure CSI feedback measurements. For example, which CSI-RSresource to measure on can be configured in a UE specific manner.Moreover, the support of antenna configurations larger than four (4)antennas must resort to CSI-RS, since the CRS is only defined for atmost four (4) antennas.

By measuring on a CSI-RS, a UE can estimate the effective channel theCSI-RS is traversing including the radio propagation channel, antennagains, and any possible antenna virtualizations (i.e., a CSI-RS port maybe precoded so that it is virtualized over multiple physical antennaports. That is, the CSI-RS port can be transmitted on multiple physicalantenna ports, possibly with different gains and phases). In moremathematical rigor, this implies that if a known CSI-RS signal x_(n) istransmitted, a UE can estimate the coupling between the transmittedsignal and the received signal (i.e., the effective channel). Therefore,if no virtualization is performed in the transmission,

y _(n) =H _(n) x _(n) +e _(n)

That is, the UE can measure the effective channel H_(eff)=H_(n).Similarly, if the CSI-RS is virtualized using a precoder W_(N) _(T)_(×r) as

y _(n) =H _(n) W _(N) _(T) _(×r) x _(n) +e _(n),

then the UE can estimate the effective channel H_(eff)=H_(n)W_(N) _(T)_(×r).

Related to CSI-RS is the concept of zero-power CSI-RS resources (alsoknown as a muted CSI-RS). Zero-power CSI-RS resources are configuredjust as regular CSI-RS resources, so that a UE knows that the datatransmission is mapped around those resources. The intent of thezero-power CSI-RS resources is to enable the network to mute thetransmission on the corresponding resources as to boost the SINR of acorresponding non-zero power CSI-RS, possibly transmitted in a neighborcell/transmission point. For LTE-Release 11, a special zero-power CSI-RSthat a UE is mandated to use for measuring interference plus noise isunder discussion. As the name indicates, a UE can assume that theTransmission Points (TPs) of interest are not transmitting on the mutedCSI-RS resource and the received power can therefore be used as ameasure of the interference plus noise level.

Based on a specified CSI-RS resource and an interference measurementconfiguration (e.g. a muted CSI-RS resource), the UE can estimate theeffective channel and noise plus interference, and consequently alsodetermine which rank, precoder and transport format to recommend thatbest match the particular channel.

Implicit CSI Feedback

For CSI feedback LTE has adopted an implicit CSI mechanism where a UEdoes not explicitly report, e.g., the complex valued elements of ameasured effective channel, but rather, recommends a transmissionconfiguration for the measured effective channel. The recommendedtransmission configuration thus implicitly gives information about theunderlying channel state.

In Releases 8 and 9 of LTE, the CSI feedback is given in terms of atransmission rank indicator (RI), a precoder matrix indicator (PMI), andchannel quality indicator(s) (CQI). The CQI/RI/PMI report can bewideband or frequency selective depending on which reporting mode thatis configured.

The RI corresponds to a recommended number of streams that are to bespatially multiplexed, and thus, transmitted in parallel over theeffective channel. The PMI identifies a recommended precoder (in acodebook) for the transmission, which relates to the spatialcharacteristics of the effective channel. The CQI represents arecommended transport block size (i.e., coderate). Thus, there is arelation between a CQI and an SINR of the spatial stream(s) over whichthe transport block is transmitted.

The implicit feedback framework has many advantages over more explicitfeedback, most notably

-   -   The UE implementation becomes, to a large extent, transparent to        the reporting mechanism and the testing thereof;    -   It encourages advanced/effective receiver implementation since        such UEs can report higher CQI and/or higher transmission rank,        and as such, immediately benefit from the added implementation        effort. Such advanced receiver designs include, but are not        limited to:        -   Increased number of UE receive antennas;        -   Advanced interference suppression techniques; and        -   Advanced channel estimation for demodulation and CSI            reporting.

Explicit CSI feedback has the disadvantage that the UE receiverimplementation is typically not included in the reporting, and itbecomes increasingly difficult for the network/UE to manage/utilizedifferent UE receiver implementations. Moreover, it is generally moredifficult to provide effective interoperability testing for such CSIfeedback mechanisms.

Note that in some contexts a CQI is interpreted to mean SINR, but thatis not the proper definition in LTE contexts. Most notably, reporting anSINR corresponds to the category of explicit CSI, whereas CQI as definedabove falls in the implicit CSI category.

Coordinated Multipoint Transmission (CoMP)

Coordinated Multipoint (CoMP) transmission and reception refers to asystem where the transmission and/or reception at multiple,geographically separated antenna sites is coordinated in order toimprove system performance. More specifically, CoMP refers tocoordination of antenna arrays that have different geographical coverageareas. In the subsequent discussion we refer to an antenna covering acertain geographical area as a point, or more specifically as aTransmission Point (TP). The coordination can either be distributed, bymeans of direct communication between the different sites, or by meansof a central coordinating node.

CoMP is a tool introduced in LTE to improve the coverage of high datarates, the cell-edge throughput and/or to increase system throughput. Inparticular, the goal is to distribute the user perceived performancemore evenly in the network by taking control of the interference in thesystem, either by reducing the interference and/or by predicting theinterference more accurately.

CoMP operation targets many different deployments, includingcoordination between sites and sectors in cellular macro deployments, aswell as different configurations of Heterogeneous deployments, where forinstance a macro node coordinates the transmission with pico nodeswithin the macro coverage area.

Further, there are many different CoMP transmission schemes that areconsidered. For example,

-   -   Dynamic Point Blanking: Dynamic Point Blanking is where multiple        TPs coordinates the transmission so that neighboring TPs may        mute the transmissions on the time-frequency resources (TFREs)        that are allocated to UEs that experience significant        interference.    -   Dynamic Point Selection: Dynamic Point Selection is where the        data transmission to a UE may switch dynamically (in time and        frequency) between different TPs, so that the TPs are fully        utilized.    -   Coordinated Beamforming: Coordinated Beamforming is where the        TPs coordinate the transmissions in the spatial domain by        beamforming the transmission power in such a way that the        interference to UEs served by neighboring TPs are suppressed.    -   Joint Transmission: Joint Transmission is where the signal to a        UE is simultaneously transmitted from multiple TPs on the same        time/frequency resource. The aim of joint transmission is to        increase the received signal power and/or reduce the received        interference (if the cooperating TPs otherwise would serve some        other UEs without taking our JT UE into consideration).

CoMP Feedback

A common denominator for the CoMP transmission schemes is that thenetwork needs CSI information not only for the serving TP, but also forthe channels linking the neighboring TPs to a terminal. For example, byconfiguring a unique CSI-RS resource per TP, a UE can resolve theeffective channels for each TP by measurements on the correspondingCSI-RS. A CSI-RS resource can loosely be described as the pattern ofresource elements on which a particular CSI-RS configuration istransmitted. A CSI-RS resource is determined by a combination of“resourceConfig”, “subframeConfig”, and “antennaPortsCount”, which areconfigured by Radio Resource Control (RRC) signaling. The UE is likelyunaware of the physical presence of a particular TP. It is onlyconfigured to measure on a particular CSI-RS resource, without knowingof any association between the CSI-RS resource and a TP.

A few candidates for CoMP feedback are on the table for LTE Release-11.Most alternatives are based on per CSI-RS resource feedback, possiblywith CQI aggregation of multiple CSI-RS resources, and possibly withsome sort of co-phasing information between CSI-RS resources. Thefollowing list briefly introduces a few relevant alternatives (note thata combination of the alternatives is also possible):

-   -   Per CSI-RS resource feedback corresponds to separate reporting        of channel state information (CSI) for each of a set of CSI-RS        resources. Such a CSI report could for example correspond to a        Precoder Matrix Indicator (PMI), Rank Indicator (RI), and/or        Channel Quality Indicator (CQI), which represent a recommended        configuration for a hypothetical downlink transmission over the        same antennas used for the associated CSI-RS (or as the RS used        for the channel measurement). More generally, the recommended        transmission should be mapped to physical antennas in the same        way as the reference symbols used for the CSI channel        measurement. Additionally, there could be interdependencies        between the CSI reports. For example, they could be constrained        to have the same RI.    -   Typically there is a one-to-one mapping between a CSI-RS and a        TP, in which case per CSI-RS resource feedback corresponds to        per-TP feedback; that is, a separate PMI/RI/CQI is reported for        each TP.    -   Additionally, the considered CSI-RS resources are configured by        an eNodeB as the CoMP Measurement Set.    -   Aggregate feedback corresponds to a CSI report for a channel        that corresponds to an aggregation of multiple CSI-RS. For        example, a joint PMI/RI/CQI can be recommended for a joint        transmission over all antennas associated with the multiple        CSI-RS.

A joint search may, however, be too computationally demanding for theUE, and a simplified form of aggregation is to evaluate an aggregate CQIand RI, which are combined with per CSI-RS resource PMIs. Such a schemealso has the advantage that the aggregated feedback may share muchinformation with a per CSI-RS resource feedback. This is beneficialbecause many CoMP transmission schemes require per CSI-RS resourcefeedback, and to enable eNodeB flexibility in dynamically selecting CoMPscheme, aggregated feedback would typically be transmitted in parallelwith per CSI-RS resource feedback. To support coherent jointtransmission, such per CSI-RS resource PMIs can be augmented withco-phasing information enabling the eNodeB to rotate the per CSI-RSresource PMIs so that the signals coherently combine at the receiver.

Interference Measurements for CoMP

For efficient CoMP operation it is equally important to captureappropriate interference assumptions when determining the CQIs as it isto capture the appropriate received desired signal. In uncoordinatedsystems the UE can effectively measure the interference observed fromall other TPs (or all other cells), which will be the relevantinterference level in an upcoming data transmission. Such interferencemeasurements are typically performed by analyzing the residualinterference on CRS resources (after the UE subtracts the impact of theCRS signal).

In coordinated systems performing CoMP, such interference measurementsbecome increasingly irrelevant. Most notably, within a coordinationcluster an eNodeB can to a large extent control which TPs that interferewith a UE in any particular TFRE. Hence, there will be multipleinterference hypotheses depending on which TPs are transmitting data toother terminals.

For the purpose of improved interference measurements, new functionalityis introduced in LTE Release-11, where the agreement is that the networkwill be able to configure which particular TFREs are to be used forinterference measurements for a particular UE. The network can thuscontrol the interference seen on those TFREs by muting all TPs within acoordination cluster on those TFREs, for example, in which case theterminal will effectively measure the inter-CoMP cluster interference.

Moreover, take for example a dynamic point blanking scheme, where thereare (at least) two relevant interference hypothesis for a particular UE.In one interference hypothesis, the UE sees no interference from thecoordinated transmission point. In the other hypothesis, the UE seesinterference from the neighboring point. To enable the network toeffectively determine whether a TP should be muted, the UE can reporttwo (or generally multiple) CQIs corresponding to different interferencehypotheses.

To facilitate such a scheme, it has been proposed to configure multipledistinct sets of interference measurement TFREs, wherein the network isresponsible for realizing each relevant interference hypothesis in oneof these sets of TFREs. Hence, by associating a particular reported CQIwith a particular set of TFREs the relevant CQIs can be made availableto the network for effective scheduling.

Alternatively, the eNodeB can perform post processing on a reported CQIas to estimate the relevant CQIs for the relevant interferencehypotheses.

In a CoMP setup, it becomes increasingly difficult for a UE toautonomously determine interference levels that are relevant for aparticular CoMP transmission hypothesis. Particularly, the UE would notknow which transmit points are muted on any particular resourceelements. Therefore, when performing an interference measurement, itwill be difficult for the UE to know exactly what is measured. This mayresult in incorrect CSI reports that do not accurately match the actualtransmission.

Moreover, the UE will not know which CoMP transmission scheme aparticular network is capable of or intends to use. Thus, a UE needs toprovide CSI reports that are relevant for numerous CoMP schemes,regardless if whether the network intends to use the information. Thisresults in unnecessarily excessive uplink overhead.

Document 3GPP Draft, R1-094141, 20091012 3rd Generation PartnershipProject (3GPP), Mobile Competence Centre, 650, route des Lucioles,F-06921 Sophia-Antipolis Cedex, France, discloses implicit feedback insupport of downlink coordinated multi-point (CoMP). The feedback isbased on either one or a combination of, amongst other, the followinghypotheses: single vs. multi user MIMO; single cell vs. coordinatedtransmission and transmit precoder. In order to support dynamic switchbetween transmission schemes, UE may report a number of CQIscorresponding to different assumptions of transmission schemes. Underthe assumption of joint transmission, if transmission points are equalto the measurement set, a single integrated CQI is then sufficient foreNB to make scheduling decisions. The UE reports CQIs calculated fromreceived powers for e.g. three cells. The reported CQIs are CQIs of eachcell with the assumption of single cell transmission. Alternatively, theUE can feed back CQIs of any of three combinations for the three cells.The eNB is still able to figure out all wanted CQIs.

Document 3GPP Draft, R1-120224, 20120131 3rd Generation PartnershipProject (3GPP), Mobile Competence Centre, 650, route des Lucioles,F-06921 Sophia-Antipolis Cedex, France, discloses CQI definition forCoMP. Assuming per-CSI-RS-resource PMI is to be used, both possibilityof aggregated CQI (derived across multiple CSI-RS-resource) andper-CSI-RS-resource CQI are included. CQI calculation includes twoparts, single power estimation and interference power estimation. Eitherof them may be measured per-CSI-RS-resource or across multipleCSI-RS-resources.

Document 3GPP Draft, R1-113892, 20111108 3rd Generation PartnershipProject (3GPP), Mobile Competence Centre, 650, route des Lucioles,F-06921 Sophia-Antipolis Cedex, France, discloses feedback operation forCoMP operation. The eNB can control which CoMP transmission hypothesesare tested by the UE by controlling an interference power offset for theinterfering points within the CoMP measurement set, restricting the setof CoMP transmission hypothesis options. Moreover, the reportedaggregated CQI is based on one of multiple CoMP transmission hypotheses.Then, the UE reports the assumed CoMP transmission hypothesis along withthe CQI.

SUMMARY

Accordingly, the present disclosure provides a system and method forimproving the link adaptation in a wireless communication system. In oneembodiment, the method is performed at a User Equipment (UE) andcomprises the UE receiving a configuration message from an eNodeB. Theconfiguration message specifies at least one Channel State Information(CSI) report that, in turn, specifies an interference hypothesis and adesired signal hypothesis that corresponds to a hypothetical datatransmission over an effective channel that is characterized by areference signal. The UE also estimates interference according to thespecified interference hypothesis, and estimates properties of theeffective channel. Based on the interference estimation and on theestimated properties of the effective channel, the UE determines atleast one CSI report, and transmits the CSI report to the eNodeB.

In another embodiment, the present disclosure provides a UE configuredto improve link adaptation in a wireless communication system. The UEcomprises a communications interface and a programmable controller. Thecommunications interface is configured to receive a configurationmessage from an eNodeB. As above, the configuration message specifies atleast one first CSI Report that, in turn specifies an interferencehypothesis and a desired signal hypothesis corresponding to ahypothetical data transmission over an effective channel that ischaracterized by a reference signal. In one embodiment, the controllerat the UE is configured to estimate interference according to thespecified interference hypothesis, as well as the properties of theeffective channel, determine at least one CSI report based on theinterference estimation and the estimated properties of the effectivechannel, and then send the at least one CSI report to the eNodeB.

In addition to a UE, the present disclosure also provides an eNodeB andcorresponding method for link adaptation in a wireless communicationsystem. In one embodiment, the method performed at the eNodeB comprisestransmitting a configuration message to a UE. In these embodiments, theconfiguration message specifies at least one Channel State Information(CSI) report specifying an interference hypothesis and a desired signalhypothesis corresponding to a hypothetical data transmission over aneffective channel that is characterized by a reference signal. Theconfiguration message configures the UE to estimate interferenceaccording to the specified interference hypothesis, as well as theproperties of the effective channel, and determine the at least one CSIreport based on the interference estimation and the estimated propertiesof the effective channel. Thereafter, the eNodeB receives, from the UE,the at least one CSI report.

To perform the method, one embodiment of the present disclosure providesan eNodeB that is configured to improve link adaptation in a wirelesscommunications system. The eNodeB comprises a controller and acommunications interface configured to transmit a configuration messageto a UE. The configuration message specifies at least one CSI reportthat specifies an interference hypothesis and a desired signalhypothesis corresponding to a hypothetical data transmission over aneffective channel characterized by a reference signal. The controller,which is operatively connected to the communications interface, isconfigured to estimate interference according to the specifiedinterference hypothesis, estimate properties of the effective channel,and determine the at least one CSI report based on the interferenceestimation and the estimated properties of the effective channel.Thereafter, the eNodeB receives the CSI report(s) from the UE.

Accordingly, the embodiments of the present disclosure provide theeNodeB with the flexibility to configure a CSI report to match aspecific CoMP transmission hypothesis, which is a candidate for adownlink transmission to said UE.

The present disclosure provides advantages that conventional systems andmethods are not able to provide. For example, the present disclosureprovides the flexibility needed for the eNodeB to configure CSIreporting only for the CoMP transmission hypotheses that are candidatesfor a subsequent transmission. This reduces uplink overhead byeliminating reporting of CSI for non-candidate CoMP transmissionhypotheses, such as CoMP transmissions the eNodeB is not capable oftransmitting, for example.

The present disclosure also provides increased flexibility for awireless network to configure CSI reports that are relevant for aparticular implementation, which is often different from any genericscheme considered for standardization. This improves the link-adaptationand downlink spectral efficiency.

Additionally, the present disclosure decreases UE processing byminimizing the number of CSI reports that a UE needs to compute, therebyreducing the draw on the battery and saving battery resources.

Further, the present disclosure decreases downlink overhead by notrequiring a network to provide interference measurement resources forinterference hypotheses that are not candidates for downlinktransmission.

Of course, those skilled in the art will appreciate that the presentdisclosure is not limited to the above contexts or examples, and willrecognize additional features and advantages upon reading the followingdetailed description and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the transmission structure ofprecoded spatial multiplexing mode in LTE.

FIG. 2 is a functional block diagram of a LTE network.

FIG. 3 is a functional block diagram of a User Equipment configuredaccording to one embodiment of the present disclosure.

FIGS. 4 and 5 are flow diagrams illustrating a method performed by theUE according to embodiments of the present disclosure.

FIG. 6 is a functional block diagram of an eNodeB configured accordingto one embodiment of the present disclosure.

FIGS. 7 and 8A-8C are flow diagrams illustrating a method performed bythe eNodeB according to embodiments of the present disclosure.

DETAILED DESCRIPTION

Turning now to the figures, a representative example of a modernwireless communication network standard is the Long Term Evolution(LTE), defined by the Third Generation Partnership Project (3GPP). FIG.2 illustrates a functional block diagram of a LTE network 10, includinga core network 12 (i.e., the evolved packet core) and a Radio Accessnetwork 14 (i.e., the Evolved Universal Terrestrial Radio AccessNetwork, or E-UTRAN). The evolved packet core network 12 comprises aplurality of nodes 16 including those having the functionality of aMobile Management Entity (MME) and a Signaling Gateway (S-GW). TheE-UTRAN nodes include evolved Node B's (eNodeB) 18 that communicativelyconnect to each other over the logical X2 interface and to the MME/SGWsnodes 16 over the logical 51 interface. Additionally, the eNodeBs 18also communicate with one or more user terminals, referred to herein asUser Equipment (UE) 20, over an air interface to provide the UEs 20 withaccess to the evolved packet core network 12.

As previously stated, the present disclosure provides a system andmethod for improving the link adaptation in a wireless communicationsystem. In one embodiment, a UE receives, from an eNodeB, aconfiguration message that specifies a CSI report. The CSI report isspecified by a particular interference hypothesis and a particulardesired signal hypothesis corresponding to data transmission over atleast one effective channel characterized by a specific referencesignal. The UE may further be configured to perform interferenceestimation according to the interference hypothesis, and/or estimate atleast one effective channel by performing measurements on the specificreference signal. Additionally, in one embodiment, the UE is configuredto determine a CSI report based on the interference estimation and theestimated effective channel, and is also configured to transmit the CSIreport to an eNodeB.

Therefore, the present disclosure provides the eNodeB with theflexibility to configure a CSI report to match a specific CoMPtransmission hypothesis, which is a candidate for a downlinktransmission to said UE.

In one exemplary embodiment a plurality of CSI reports are configured,wherein the eNodeB configures said CSI reports to match a plurality ofcorresponding CoMP transmission hypothesis. In another embodiment, aneNodeB can also configure the number of the CSI reports. Suchembodiments are useful in the context of CoMP, where an eNodeB iscapable of coordinated transmissions from multiple transmission points,and the eNodeB needs CSI for each of multiple hypotheses of coordinatedtransmissions (e.g., wherein a neighbouring point is muted or not muted,or wherein a neighbouring point is participating in the datatransmission or not).

In another embodiment, a desired signal hypothesis for a specific CSIreport is configured by signalling, from which a UE can determine abitmap. Each bit is associated with one of a plurality of referencesignals, and the value of each bit specifies whether a UE should assume,for the specific CSI report, that the desired signal is transmitted overthe effective channel identified by the reference signal associated withthe bit. The advantage of this embodiment is that the eNodeB is providedfull flexibility to configure reporting of aggregated CQIs (as well asper-TP CQIs). If multiple bits indicate a desired signal then the UEdetermines a CSI report with the associated aggregated CQI correspondingto a joint transmission.

In another embodiment, an eNodeB can configure the signal hypothesis (orthere can be a predetermined contract) such that whenever two or morebits in the bitmap indicate a desired signal on the two or moreassociated effective channels, the specific UE should assume for the CSIreport that the eNodeB transmits a desired signal incoherently betweenthe two or more effective channels. The advantage of this embodiment isthat it is often demanding for a network to guarantee a coherenttransmission from multiple transmission points. Particularly, therelative phases between two effective channels (associated with the twotransmission points) may change substantially between the point the CSIreport is determined/estimated and the time of an actual transmissionthat follow the CSI report. In these cases it is often better totransmit using an incoherent transmission scheme, wherein the linkadaptation will be improved if the UE assumes the same incoherenttransmission scheme, for example, the CQI reporting.

In another embodiment, an eNodeB can configure the signal hypothesis (orthere can be a predetermined contract) such that a specific pattern offrequency selective relative phase shifts (which could be static, orfully or partially pseudo random) should be applied to the transmissionsbetween the two or more effective channels. By randomly or structurallyimposing frequency selective relative phase shifts for the transmissionsbetween the different transmit points, the transmission can beguaranteed to have incoherent frequency selective relative phase shiftsfor maximum diversity in the combining of signals from the differenttransmit points.

In another embodiment, an eNodeB can configure the signal hypothesis (orthere can be a predetermined contract) such that whenever two or morebits in the bitmap indicate a desired signal, the specific UE shouldassume for the CSI report that the eNodeB transmits a desired signalcoherently over the plurality of associated effective channels.

In another embodiment, the assumed transmitted signal is transmittedusing specific wideband relative phase shifts among each such effectivechannel. A special case is that each such relative phase is zeroradians. The advantage with such a convention is that there will not beany need to signal any phase information for the transmissions betweenseparate transmission points, since the CQI and other elements of theprecoder report will be conditioned on a specific set of relative phases(that are also known by the eNodeB). The UE can therefore report per TPPMIs (typically restricted to be of the same rank) which can be used toform the recommended transmission by the network. More specifically,even a fixed phase configuration the randomness of the effectivechannels over frequency will ensure that with high probability therewill be at least some subbands in which the effective channels match thefixed relative phases. Thus, an eNodeB can select to transmit to theparticular UE on these particularly accurately matched subbands, andpossibly allocate the remaining (ill-matched) subbands to other UEs.

In another embodiment, the CSI report further comprises a recommendedaggregate precoder that includes recommended relative phase informationfor transmissions over the plurality of effective channels. In suchembodiments, other elements of the CSI report assume that an eNodeBtransmits according to the recommended aggregate precoder. The advantagewith this embodiment is that the UE can explicitly recommend how toco-phase the transmissions from separate transmission points. Forexample, if this information is provided at a per-subband granularity,then the eNodeB is provided with information on how to transmit withconstructive coherence on all subbands.

In another embodiment, an aggregated CQI is reported assuming an eNodeBtransmits according to the recommended aggregate precoder.

In another embodiment, there is a contract between the UE and eNodeBthat no CSI reports correspond to joint transmission. In suchembodiments, the bitmap can be derived from an index that indicateswhich of the plurality of reference signals corresponding to the singleeffective channel over which the desired signal is assumed to betransmitted. Further, such an index is explicitly or implicitlyconfigured by an eNodeB for the specific CSI report. This embodiment hasadvantage that if no CSI reports corresponding to joint transmission areneeded by the network, then the downlink overhead can be reduced since afull bitmap does not need to be signalled. Instead, only an indexspecifying which bit in the bitmap is non-zero needs to be signalled.Even if the system supports joint transmissions, an eNodeB can to alarge extent derive the required CSI from multiple per-TP CSI reports.

In another embodiment, there is a hierarchical ordering among aplurality of CSI reports. More specifically, the configuration of aspecific CSI report requires the presence of at least one other secondCSI report. This embodiment may be useful because it enables dependencebetween CSI reports which can reduce computational complexity andreporting overhead. Moreover, configuring the first CSI report couldautomatically trigger the reporting the second CSI report therebyreducing configuration overhead.

In another embodiment, the specific CSI report reuses elementsdetermined for the second CSI report. This embodiment is useful when thefeedback overhead and/or UE computational complexity is taken intoaccount. Particularly, some information can be shared between multiplereports, and therefore, only determined once. Practical useful examplesinclude, but are not limited to, situations in which per TP PMIrecommendations have been derived for a set of single pointtransmissions. In such cases, the PMIs are simply reused for a jointtransmission hypothesis among these transmission points.

In another embodiment, a predetermined contract exists between an eNodeBand the specific UE associating a predetermined desired signalhypothesis with each of a plurality of CSI reports. One of theadvantages of specifying (e.g., as part of the standard) that each CSIreport will assume a specific desired signal transmission hypothesis isthat the overhead is minimized. Additionally, a UE implementation maytake advantage of this knowledge in the implementation to optimizeperformance. With this embodiment, the eNodeB only needs toensure/configure that the UE is assuming the correct interferencehypothesis for each CSI report. Examples of such predetermined contractsinclude embodiments in which the n:th CSI report assumes a desiredsignal over the effective channel associated with the n:th referencesignal in a CoMP Measurement Set (which may be separately configured).

In another embodiment, the UE is configured to use a specific set oftime-frequency resource elements for an interference measurement onwhich the UE bases the particular interference hypothesis for thespecific CSI report. This embodiment has the advantage that the eNodeBcan configure a pattern of TFREs (e.g., a zero power CSI-RS resource, ora non-zero power CSI-RS) on which the terminal measures theinterference. Thus, the eNodeB can configure a pattern on which theinterference closely corresponds to what is seen in a CoMP transmissioncorresponding to the hypothesis assumed for the CSI report. For example,the UE can mute any data from a neighbouring point.

In another embodiment, a contract exists between an eNodeB and a UEregarding a reference resource for which the UE autonomously performs aninterference measurement, on which the UE may base the particularinterference hypothesis for the specific CSI report. The advantage ofthis embodiment is that it minimizes the configuration overhead sincethe UE itself determines a relevant interference measurement for the CSIreport. However, with such a scheme, it may be difficult for the networkto predict what interference was included in the report.

In another embodiment, an eNodeB further configures an interferencehypothesis for the specific CSI report. By way of example, the eNodeBmay signal the UE to amend the interference measurement by artificiallyadding interference from at least one virtual interfering transmissionover an effective channel characterized by a reference signal that isidentified by the configuration. The advantage of this embodiment isthat interference which may be difficult to measure (e.g., interferencethat is not transmitted on any pattern of TFREs) can be included in aninterference hypothesis. Instead of having the terminal passivelymeasure an interference level (or covariance matrix), the UE willactively estimate the interference for a particular transmit point. Forexample, the UE may assume that an isotropic signal of a certain power(could be predetermined or configured) is transmitted over a measuredeffective channel, and add (inject) this interference to the (passive)interference measurement. In particular, for large CoMP coordinationclusters, it becomes increasingly difficult (and the overhead becomesincreasingly excessive) for the network to actively construct patternsof TFREs corresponding to every relevant interference hypothesis in thecoordination cluster. Therefore, this embodiment can alleviate thenetwork overhead by having multiple interference hypotheses share acommon interference denominator in a shared pattern of TFREs, and byhaving the UE artificially inject the distinguishing interference foreach individual interference hypothesis.

In another embodiment, the interference hypothesis is configured bysignalling from which a second bitmap can be determined by a UE. In thisembodiment, each bit is associated with one out of a second plurality ofreference signals, and the value of each bit specifies whether a UEshould amend the interference measurement by artificially addinginterference from a virtual transmission over the effective channelcharacterized by the reference signal associated with said bit. Theadvantage of this embodiment is that the eNodeB is provided with thefull flexibility to configure a UE to construct the interferencehypothesis by adding all or some interfering sources to the interferencehypothesis.

In another embodiment, none of the bits of the second bitmap isassociated with a reference signal that corresponds to an effectivechannel that is assumed for a desired signal transmission for thespecific CSI hypothesis. The advantage of this embodiment is thatconfiguration overhead can be reduced by observing that a signal cannotbe both interference and a desired signal. Thus, having an interferencetriggering bit associated with a desired signal is redundant. This canbe used, to reduce the overhead.

In another embodiment, the plurality of reference signals and/or secondplurality of reference signals are channel state information referencesignals (CSI-RS) configured in a CoMP Measurement Set.

In another embodiment, an eNodeB configures the specific UE (or there isa predetermined contract with the UE) with a list of possibleinterference hypotheses, and/or a particular desired signal hypothesis,and/or pairs thereof, from which the eNodeB configures the specific CSIreport by signalling an index to an element in said list. Thisembodiment has the advantage that it can achieve reduced configurationoverhead and a simpler UE implementation by restricting the possibleinterference/desired signal hypotheses to a predetermined set for whichthe implementation can be targeted. Moreover, this embodiment providesthe possibility to actively eliminate irrelevant interference/desiredsignal combinations, and thereby reduces the overhead.

In another embodiment, an eNodeB configured according to the presentdisclosure acquires CSI reports for a plurality of CoMP transmissionhypotheses for transmission points associated with reference signalsbelonging to a CoMP Measurement Set that is configured for a specificUE.

In one embodiment, the eNodeB mutes the transmission points on aspecific set of TFREs, and configures the specific UE to use the set ofTFREs for interference measurements for at least one specific CSIreport.

In another embodiment, the eNodeB configures the specific CSI report tocorrespond to a dynamic point blanking hypothesis. In such embodiments,a first transmission point is transmitting a desired signal, and atleast a second transmission point is muted, by configuring the CSIreport to associate the desired signal with a single reference signalcorresponding to the first transmission point. Additionally, the eNodeBconfigures an interference hypothesis not including interference from atleast the second transmission point.

In another embodiment, configuring the interference hypothesis furthercomprises configuring the UE to artificially add interference from atleast one third transmission point by signalling to the UE an index (orbitmap) identifying a reference signal transmitted from the thirdtransmission point, and to inform the UE that the interferencemeasurement should be amended with virtual interference transmitted overthe effective channel associated with the reference signal.

In another embodiment, the eNodeB configures the specific CSI report tocorrespond to a single point transmission hypothesis. In suchembodiments, a transmission point transmits a desired signal byconfiguring the CSI report to associate the desired signal with a singlereference signal corresponding to the transmission point. Moreover, theeNodeB configures an interference hypothesis not including interferencefrom the transmission point.

In another embodiment, configuring the interference hypothesis furthercomprises configuring the UE to artificially add interference from atleast one second transmission point by signalling to the UE an index (orbitmap) identifying a reference signal transmitted from the transmissionpoint, and informing the UE that the interference measurement should beamended with virtual interference transmitted over the effective channelassociated with said reference signal.

In another embodiment, the eNodeB configures the specific CSI report tocorrespond to a joint transmission hypothesis in which a plurality oftransmission points are transmitting a desired signal, by configuringthe CSI report to associate the desired signal with a plurality ofreference signals corresponding to the plurality of transmission points.Moreover, in this embodiment, the eNodeB may configure an interferencehypothesis not including interference from at least the plurality oftransmission points.

In one embodiment, configuring the interference hypothesis furthercomprises configuring the UE to artificially add interference from atleast one transmission point that is not in the set of the plurality oftransmission points associated with desired signals. This may beaccomplished, for example, by signalling to the UE an index (or bitmap)identifying a reference signal transmitted from said transmission point,and informing the UE that the interference measurement should be amendedwith virtual interference transmitted over the effective channelassociated with said reference signal.

In another embodiment, the eNodeB configures the specific CSI report toreuse the rank indicator from a second CSI report corresponding to asingle point transmission hypothesis, and/or a dynamic point blankinghypothesis corresponding to a desired signal transmitted from one of theplurality of transmission points.

In one embodiment, the eNodeB configures the specific CSI report toreuse the per point precoder matrix indicators from a plurality of CSIreports corresponding to single point transmission hypotheses and/ordynamic point blanking hypotheses. In such embodiments, each of theplurality of CSI reports corresponds to a desired signal transmittedfrom one of the plurality of transmission points in the jointtransmission hypothesis. Further, each of the said plurality of CSIreports is restricted to the same rank as the said joint transmissionhypothesis. Additionally, each of the plurality of CSI reportscorresponds to a unique signal transmission point within the pluralityof transmission points associated with the joint transmissionhypothesis.

FIG. 3 is a functional block diagram illustrating some components of anexemplary UE 20 configured to operate according to one or moreembodiments of the present disclosure. As seen in FIG. 3, UE 20comprises a programmable controller 22, a memory 24, a user I/Ointerface 26, and a communications interface 28. The user I/O interface26 provides the components necessary for a user to interact with the UE20. The communications interface 28 comprises a transceiver thatfacilitates the communications with the eNodeBs 18 of the E-UTRAN overthe appropriate air interface. In one embodiment, the communicationsinterface communicates signals and data with the eNodeB s 18 inaccordance with the LTE standards. The memory 24 may comprise any solidstate memory or computer readable media known in the art. Suitableexamples of such media include, but are not limited to, ROM, DRAM,Flash, or a device capable of reading computer-readable media, such asoptical or magnetic media.

The programmable controller 22 may be implemented by one or moremicroprocessors, hardware, firmware, or a combination thereof, andgenerally controls the operation and functions of the UE 20 according tothe appropriate standards. Such operations and functions include, butare not limited to, communicating with the eNodeBs 18 as previouslydescribed in this application. In this regard, the programmablecontroller 22 may be configured to implement logic and instructionsstored in memory 24 to perform the method of the present disclosure toimprove the link adaptation.

FIG. 4 is a flow diagram illustrating a method 30 performed by a UE 20according to one embodiment of the present disclosure. Method 30 beginswith the UE 20 receiving a configuration message from an eNodeB (box32). The configuration message specifies at least one Channel StateInformation (CSI) report that specifies an interference hypothesis, aswell as a desired signal hypothesis that corresponds to a hypotheticaldata transmission over an effective channel characterized by a referencesignal. The UE 20 then estimates interference according to the specifiedinterference hypothesis and the estimating properties of the effectivechannel (box 34), and determines at least one CSI report based on theinterference estimation and the estimated properties of the effectivechannel (box 36). Once determined, the UE 20 transmits the CSI report tothe eNodeB (box 38).

In this embodiment, the configuration message may, for example, specifya CSI process with which the CSI report is associated. Further, in oneembodiment, the interference hypothesis is specified, at least in part,by a Channel State Information-Interference Measurement (CSI-IM)configuration, while in another embodiment, the desired signalhypothesis is specified by a Channel State Information-Reference Signal(CSI-RS) configuration. In one embodiment, however, both theinterference hypothesis and the desired signal hypothesis are specified,at least in part, by respective CSI-IM and CSI-RS configurations.

FIG. 5 illustrates a method 40 in which the UE 20 generates the CSIreport according to one embodiment. As seen in FIG. 5, the UE 20determines a bitmap for the CSI report from the configuration message(box 42). Each bit in the bitmap is associated with one of a pluralityof reference signals, and each reference signal is associated with adifferent effective channel. Then, based on a value of a given bit, theUE 20 determines whether at least parts of the hypothetical datatransmission is assumed transmitted over the effective channelidentified by the reference signal associated with the given bit (box44). Then, for each component of the hypothetical data transmission, theUE 20 determines whether that component is assumed to have beentransmitted coherently, incoherently, or on a single effective channelbased on a predetermined contract, or on information in theconfiguration message (box 46).

The UE 20 may further determine a second bitmap based on signals sent bythe eNodeB (box 48). In one embodiment, the UE 20 determines the secondbit map such that each bit in the second bitmap has a value and isassociated with a corresponding one of a second plurality of referencesignals. Further, each reference signal corresponds to an effectivechannel. In such cases, the UE 20 may determine, based on the value ofthe bits in the second bitmap, whether to modify the interferencemeasurement by artificially adding an interference measurement from avirtual transmission over the effective channel identified by thereference signal associated with the bit (box 50).

It should be noted that one or both of the plurality of referencesignals and the second plurality of reference signals comprise CSI-RSconfigured in a Coordinated Multi-Point (CoMP) Measurement Set.

Continuing with FIG. 5, the configuration message, or a furtherconfiguration message, received at the UE 20 may further specify asecond CSI report that corresponds to a second desired signalhypothesis, and a second interference hypothesis. In such cases, the UE20 may, in one embodiment, configure the CSI report to reuse a rankindicator computed according to the second CSI report (box 52). Asabove, the further configuration message specifies a further CSI processwith which the second CSI report is associated. Hence, differentconfiguration messages e.g. Radio Resource Control (RRC) messages,received by the UE 20 from the eNB, may specify different CSI reportsthereby enabling the UE to provide different CSI reports independentlyof each other.

In another embodiment, the UE 20 may configure the CSI report to reuse aper point precoder matrix indicator computed according to a plurality ofCSI reports (box 54). In these latter cases, each of the plurality ofCSI reports corresponds to a desired signal transmitted from one of aplurality of transmission points in a joint transmission hypothesis, isrestricted to a same rank as the joint transmission hypothesis, andcorrespond to a unique signal transmission point within the plurality oftransmission points associated with the joint transmission hypothesis.

FIG. 6 is a functional block diagram of some components of an exemplaryeNodeB 18 configured according to one embodiment of the presentdisclosure. As shown in FIG. 4, the eNodeB 18 comprises a programmablecontroller 60, a communications interface 62, and a memory 64. Thecommunications interface 62 may, for example, comprise a transmitter andreceiver configured to operate in an LTE system or other similar system.As is known in the art, the transmitter and receiver are coupled to oneor more antennas (not shown) and communicate with the UE 20 over theLTE-based air interface. Memory 64 may comprise any solid state memoryor computer readable media known in the art. Suitable examples of suchmedia include, but are not limited to, ROM, DRAM, Flash, or a devicecapable of reading computer-readable media, such as optical or magneticmedia.

The programmable controller 60 controls the operation of the eNodeB 18in accordance with the LTE standard. The functions of the controller 60may be implemented by one or more microprocessors, hardware, firmware,or a combination thereof, and include performing the functionspreviously described. Thus, the controller 60 may be configured toaccording to logic and instructions stored in memory 64 to communicatewith the UE 20, as well as to improve the link adaptation using themethod previously described.

FIG. 7 is a flow diagram that illustrates a method 70 of performing anembodiment of the present disclosure at the eNodeB 18. Method 70 beginswith the eNodeB 18 transmitting a configuration message to a UE 20 (box72). The eNodeB 18 transmits the configuration message to configure theUE to determine the CSI report according to the previously describedembodiments.

In one embodiment, the configuration message specifies at least one CSIreport specifying an interference hypothesis and a desired signalhypothesis that corresponds to a hypothetical data transmission over aneffective channel characterized by a reference signal. The eNodeB 18transmits the configuration message to configure the UE 20 to estimateinterference according to the specified interference hypothesis, toestimate properties of the effective channel, and to determine the atleast one CSI report based on the interference estimation and theestimated properties of the effective channel. Thereafter, the eNodeB 18receives the CSI report from the UE 20 (box 74).

As above, the configuration message may specify a CSI process with whichthe CSI report is associated, and further, may specify one or both ofthe interference hypothesis and the desired signal hypothesis, at leastin part, by a CSI-IM, configuration, and a CSI-RS configuration,respectively.

FIGS. 8A-8C are flow diagrams illustrating a method 80 for performingembodiments of the present disclosure at the eNodeB 18. For example, theeNodeB 18 may, in one embodiment, configure a plurality configurationmessages to send to the UE (box 82). Each configuration messagespecifies a CSI report and is configured to match a correspondingcoordinated multi-point (CoMP) scheme that is a candidate for a downlinktransmission to the UE 20.

In another embodiment, the eNodeB 18 may configure the CSI report tocomprise a bitmap having a plurality of bits (box 84). Each bit would beassociated with one of a plurality of reference signals, and eachreference signal would be associated with a different effective channel.Further, each bit would have a corresponding value configured toindicate to the UE that a desired signal is transmitted over theeffective channel identified by the reference signal associated with thebit. The eNodeB 18 would then set two or more bits in the bitmap toindicate the transmission of desired signals on two or more effectivechannels (box 86). The two or more bits could indicate to the UE 20whether the desired signals are transmitted coherently or incoherentlybetween the two or more effective channels, based on a predeterminedcontract or on information in the configuration message.

Additionally, the eNodeB 18 could configure a plurality ofhierarchically-ordered CSI reports in which the configuration for anygiven CSI report is based on at least one other CSI report (box 88). Forexample, in such scenarios, the eNodeB 18 may configure the given CSIreport using selected information from a previous CSI report (box 90).

As seen in FIG. 8B, the eNodeB 18 may, in one embodiment, also configurethe interference hypothesis by signaling the UE 20 to modify aninterference measurement (box 92). Particularly, the eNodeB 18 maysignal the UE 20 to add an interference measurement from at least onevirtual interfering transmission over an effective channel characterizedby a reference signal that is identified by the configuration. TheeNodeB 18 may then indicate to the UE 20 how a second bitmap can bedetermined by the UE 20 (box 94). Particularly, each bit is to beassociated with one of a second plurality of reference signals. Thevalue of each bit indicates whether the UE 20 should add an interferencemeasurement from a virtual transmission over the effective channelcharacterized by the reference signal associated with a given bit in thesecond bitmap to modify the interference measurement. Further, one orboth of the plurality of reference signals and the second plurality ofreference signals comprise CSI-RS configured in a coordinatedmulti-point (CoMP) measurement set.

In one embodiment, the eNodeB 18 configures the UE 20 with a list of oneor both of the possible interference hypotheses and the desired signalhypothesis, or pairs of possible interference and desired signalhypotheses (box 96). From this information, the eNodeB 18 may configurethe CSI report by signalling an index to an element in the list, forexample.

In another embodiment, the eNodeB 18 may configure CSI reports for aplurality of CoMP transmission hypotheses for transmission points (TPs)associated with reference signals associated with a CoMP Measurement Setconfigured for the UE (box 98).

Additionally, turning to FIG. 8C, the eNodeB 18 may, in someembodiments, mute the TPs on a given set of time-frequency resources(TFREs), and configure the UE 20 to use the set of TFREs forinterference measurements for at least one CSI report (box 100).Thereafter, the eNodeB 18 may configure the CSI report to correspond toa dynamic point blanking hypothesis so that a first transmission pointtransmits a desired signal, and so that a second transmission point ismuted (box 102). In such embodiments, configuring the CSI report maycomprise, for example, the eNodeB 18 configuring the CSI report toassociate the desired signal with a single reference signal thatcorresponds to the first transmission point (box 104), and alsoconfiguring an interference hypothesis to omit information regardinginterference from at least the second transmission point (box 106).

In one embodiment, the eNodeB 18 may configure the CSI report to reuse arank indicator from a CSI report (box 108). The rank indicatorcorresponds to one or both of a single point transmission hypothesis anda dynamic point blanking hypothesis. Each of the hypotheses correspondsto a desired signal transmitted from one of the plurality oftransmission points.

In another embodiment, the eNodeB 18 configures the CSI report to reusea per point precoder matrix indicator from a plurality of CSI reportsthat correspond to one or both of a single point transmission hypothesesand a dynamic point blanking hypotheses (box 110). In these cases, eachof the plurality of the CSI reports correspond to a desired signaltransmitted from one of the plurality of transmission points in thejoint transmission hypothesis, are restricted to the same rank as thejoint transmission hypothesis, or correspond to a unique signaltransmission point within the plurality of transmission pointsassociated with the joint transmission hypothesis.

The present disclosure may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the disclosure. For example, the present disclosurealso includes the embodiments described in Appendix A. Additionally,although terminology from 3GPP LTE has been used in this disclosure toexemplify embodiments of the disclosure, those of ordinary skill in theart will readily appreciate that this if for illustrative purposes only,and that the present disclosure is not limited in scope to only theaforementioned system. Other wireless systems, including, but notlimited to, WCDMA, WiMax, UMB and GSM, may also benefit from using themethods described herein.

Additionally, note that terminology such as eNodeB and UE is alsonon-limiting, and further, does not imply any particular hierarchicalrelation between the two. Generally, an “eNodeB” could be considered asa first device and an “UE” could be considered as a second device thatcommunicates with each other over some radio channel. Further, althoughthe description specifically focuses on wireless transmissions in thedownlink, this is for illustrative purposes only. Those skilled in theart will readily appreciate that the present disclosure is equallyapplicable to wireless transmissions on the uplink.

Therefore, those of ordinary skill in the art will readily appreciatethat the present embodiments is not limited by the foregoing discussion.Nor is it limited by the accompanying figures. Rather, the presentdisclosure is limited only by the following claims and their reasonablelegal equivalents.

1. A method for improving link adaptation in a wireless communicationssystem, the method performed at a User Equipment (UE) and comprising:receiving a configuration message from an eNodeB, wherein theconfiguration message specifies at least one interference hypothesis andat least one desired signal hypothesis corresponding to a hypotheticaldata transmission over an effective channel that is characterized by areference signal, where the interference hypothesis and the desiredsignal hypothesis are specified, at least in part, by a Channel StateInformation-Interference Measurement (CSI-IM) configuration, and aChannel State Information-Reference Signal (CSI-RS) configuration,respectively; estimating interference according to the specifiedinterference hypothesis, and estimating properties of the effectivechannel; determining at least one CSI report based on the interferenceestimation and the estimated properties of the effective channel; andtransmitting the at least one CSI report to the eNodeB.
 2. The method ofclaim 1 wherein the configuration message specifies a CSI process withwhich the at least one CSI report is associated.
 3. The method of claim1, further comprising: determining a bitmap for the at least one CSIreport from the configuration message wherein each bit in the bitmap isassociated with one of a plurality of reference signals, and whereineach reference signal is associated with a different effective channel;determining, based on a value of a given bit, whether, at least parts ofthe hypothetical data transmission is assumed transmitted over theeffective channel identified by the reference signal associated with thegiven bit; and determining, for each component of the hypothetical datatransmission, whether the component is assumed to have been transmittedcoherently, incoherently, or on a single effective channel based on apredetermined contract or on information in the configuration message.4. The method of claim 1, further comprising: determining a secondbitmap based on signals sent by the eNodeB, each bit in the secondbitmap having a value and being associated with a corresponding one of asecond plurality of reference signals, and wherein each reference signalcorresponds to an effective channel; and determining, based on the valueof the bits in the second bitmap, whether to modify the interferencemeasurement by artificially adding an interference measurement from avirtual transmission over the effective channel identified by thereference signal associated with the bit.
 5. The method of claim 1wherein one or both of the plurality of reference signals and a secondplurality of reference signals comprise CSI-RS configured in aCoordinated Multi-Point (CoMP) Measurement Set.
 6. The method of claim1, wherein the configuration message, or a further configurationmessage, further specifies a second CSI report corresponding to a seconddesired signal hypothesis, and a second interference hypothesis.
 7. Themethod of claim 6 further comprising configuring the at least one CSIreport to reuse a rank indicator computed according to the second CSIreport.
 8. The method of claim 7 further comprising configuring the atleast one CSI report to reuse a per point precoder matrix indicatorcomputed according to a plurality of CSI reports, and wherein each ofthe plurality of CSI reports: correspond to a desired signal transmittedfrom one of a plurality of transmission points in a joint transmissionhypothesis; are restricted to a same rank as the joint transmissionhypothesis; and correspond to a unique signal transmission point withinthe plurality of transmission points associated with the jointtransmission hypothesis.
 9. A User Equipment (UE) configured forimproving link adaptation in a wireless communications system, the UEcomprising: a communications interface configured to receive aconfiguration message from an eNodeB, wherein the configuration messagespecifies at least one interference hypothesis and at least one desiredsignal hypothesis corresponding to a hypothetical data transmission overan effective channel that is characterized by a reference signal, wherethe interference hypothesis and the desired signal hypothesis arespecified, at least in part, by a Channel State Information-InterferenceMeasurement (CSI-IM) configuration, and a Channel StateInformation-Reference Signal (CSI-RS) configuration, respectively; and acontroller configured to: estimate interference according to thespecified interference hypothesis and estimate the properties of theeffective channel; determine at least one CSI report based on theinterference estimation and the estimated properties of the effectivechannel; and send the at least one CSI report to the eNodeB.
 10. The UEof claim 9 wherein the configuration message specifies a CSI processwith which the at least one CSI report is associated.
 11. The UE ofclaim 9, wherein the controller is further configured to: determine abitmap for the at least one CSI report from the configuration message,wherein each bit in the bitmap is associated with one of a plurality ofreference signals, and wherein each reference signal is associated witha different effective channel; determine, based on a value of a givenbit, whether, at least parts of the hypothetical data transmission isassumed transmitted over the effective channel identified by thereference signal associated with the given bit; and determine, for eachcomponent of the hypothetical data transmission, whether the componentis assumed to have been transmitted coherently, incoherently, or on asingle effective channel based on a predetermined contract or oninformation in the configuration message.
 12. The UE of claim 9, whereinthe controller is further configured to: determine a second bitmap basedon signals sent by the eNodeB, each bit in the second bitmap having avalue and being associated with a corresponding one of a secondplurality of reference signals, and wherein each reference signalcorresponds to an effective channel; and determine, based on the valueof the bits in the second bitmap, whether to modify the interferencemeasurement by artificially adding an interference measurement from avirtual transmission over the effective channel identified by thereference signal associated with the bit.
 13. The UE of claim 9 whereinone or both of the plurality of reference signals and a second pluralityof reference signals comprise CSI-RS configured in a coordinatedmulti-point (CoMP) Measurement Set.
 14. The UE of claim 9 wherein theconfiguration message, or a further configuration message, specifies asecond CSI report corresponding to a second desired signal hypothesis,and a second interference hypothesis.
 15. The UE of any of claim 14wherein the controller is further configured to configure the at leastone CSI report to reuse a rank indicator computed according to thesecond CSI report.
 16. The UE of claim 15 wherein the controller isfurther configured to configure the at least one CSI report to reuse aper point precoder matrix indicator computed according to a plurality ofCSI reports in which each CSI report: corresponds to a desired signaltransmitted from one of a plurality of transmission points in a jointtransmission hypothesis; is restricted to a same rank as the jointtransmission hypothesis; and corresponds to a unique signal transmissionpoint within the plurality of transmission points associated with thejoint transmission hypothesis.
 17. A method for improving linkadaptation in a wireless communications system, the method performed atan eNodeB and comprising: transmitting a configuration message to a UserEquipment (UE) the configuration message specifying at least oneinterference hypothesis and at least one desired signal hypothesiscorresponding to a hypothetical data transmission over an effectivechannel that is characterized by a reference signal, where theinterference hypothesis and the desired signal hypothesis are specified,at least in part, by a Channel State Information-InterferenceMeasurement (CSI-IM) configuration, and a Channel StateInformation-Reference Signal (CSI-RS) configuration, respectively, toconfigure the UE to: estimate interference according to the specifiedinterference hypothesis; estimate properties of the effective channel;and determine the at least one CSI report based on the interferenceestimation and the estimated properties of the effective channel; andreceiving, from the UE, the at least one CSI report.
 18. The method ofclaim 17 wherein the configuration message specifies a CSI process towhich the at least one CSI report is associated.